The present invention relates, in particular, to an optical pickup for generating focus error information by using an astigmatism system and a method for manufacturing the optical pickup.
In an optical pickup used in an optical disk reproducing apparatus or the like, not only information recorded on a disk is reproduced from reflected light information, but also error information such as focus, tracking or the like for precise recording/reproducing scan of a laser beam can be obtained.
In particular, error information (focus error signal) for executing a focus servo which brings the focus position of a laser light into an in-focus state with respect to a recording face of a disk or the like is obtained by means of an astigmatism system using the output of a quadruple photodetector. Such a configuration is known.
FIG. 1 shows an example of a configuration of an optical pickup disk 20.
This optical pickup 20 includes a laser diode 22, a collimator lens 23, a beam splitter 24, an objective lens 25, a condensing lens 26, a cylindrical lens 27, a photodetector 28, and a biaxial mechanism 29.
A laser beam output from the laser diode 22 is converted to a parallel beam by the collimator lens 23, then reflected toward a disk 90 by 90 degrees by the beam splitter 24, and irradiated on the disk 90 via the objective lens 25.
The objective lens 25 is held by the biaxial mechanism 29 so as to be able to move in the focus direction and the tracking direction. The operations for moving the objective lens 25 in the focus direction and the tracking direction are executed by currents applied to a focus coil and a tracking coil in the biaxial mechanism 29.
On the, disk 90, grooves GB are formed as recording tracks. However, both lands LD and grooves GB can be used as the data recording tracks.
A reflected light resulting from reflection on the disk 90 enters the beam splitter 24 via the objective lens 25. The reflected light is then transmitted through the beam splitter 24 as it is and arrives at the condensing lens 26. The reflected light is condensed by the condensing lens 26, and then is incident on the photodetector 28 via the cylindrical lens 27. As the photodetector 28, a quadruple detector having light receiving faces A, B, C and D as shown in FIGS. 2A to 2C is provided.
The cylindrical lens 27 is disposed so as to have its mother line inclined by 45 degrees with respect to the track direction of the disk 90. By utilizing the astigmatism generated by the cylindrical lens 27, the focus error signal is detected from the output of the quadruple detector.
When the beam spot is in the in-focus state with respect to the recording face of the disk 90, a spot SP on the quadruple detector becomes a circle as shown in FIG. 2A.
If the objective lens 25 is located too near the disk 90 as compared with the position of the in-focus state, however, the spot SP on the quadruple detector becomes an ellipse having its longer radius in a direction parallel to the mother line direction of the cylindrical lens 27 (i.e., direction directed from the light receiving face B toward the light receiving face D) as shown in FIG. 2B. On the contrary, if the objective lens 25 is located too far from the disk 90 as compared with the position of the in-focus state, the spot SP on the quadruple detector becomes an ellipse having its longer radius in a direction parallel to a direction perpendicular to the direction of the mother line of the cylindrical lens 27 (i.e. direction directed from the light receiving face A toward the light receiving face C) as shown in FIG. 2C.
Denoting outputs corresponding to quantities of light received by the light receiving faces A, B, C and D respectively by SA, SB, SC and SD, therefore, a focus error signal FE can be obtained as
FE=(SA+SC)-(SB+SD).
In other words, if (SA+SC)-(SB+SD) is zero, it can be detected that the objective lens 25 is in the just focus state. If (SA+SC)-(SB+SD) is a positive value, it can be detected that the objective lens 25 is located further apart from the disk 90 than the in-focus position. If (SA+SC)-(SB+SD) is a negative value, it can be detected that the objective lens 25 is located nearer the disk 90 than the in-focus position.
By constructing the focus servo system so as to converge the focus error signal FE toward zero, therefore, the focus position of the objective lens 25 can be controlled properly.
On the recording face of the recording medium, however, a series of pits formed of concave-convex pits or lands/grooves as in the above described disk 90 are formed. The irradiated laser light is modulated by them. As a result, the intensity pattern of the spot on the quadruple detector is changed. For example, the light intensity of the light spot on the receiving portions A and C becomes intense. Or on the contrary, the light intensity of the light spot on the receiving portions B and D becomes intense.
Therefore, there occurs such a phenomenon that the focus error signal FE becomes plus in, for example, the grooves GB, and becomes minus in the lands LD (or vice versa).
FIG. 3A shows an example of the focus error signal FE obtained when the laser spot traverses the grooves GB/lands LD while being kept in the in-focus state.
In the in-focus state, the focus error signal should originally become constant (zero) irrespective of the grooves GB/lands LD (or irrespective of concave-convex pit train). Due to modulation conducted by the grooves GB/lands LD, the focus error signal FE becomes a signal having an offset GFOF caused by the grooves GB and an offset LFOF caused by the lands LD as illustrated.
In other words, the relation that the focus error signal FE=0 does not necessarily indicate the in-focus state. Unless some countermeasure is taken, the focus servo does not function satisfactorily.
For example, in the case of such a system that information recording and reproducing are conducted only for either the grooves GB or the lands LD, the focus error signal FE is provided with a bias so as to cancel either the offset GFOF caused by the grooves GB or the offset LFOF caused by the lands LD. By doing so, the focus servo system converging to the in-focus state with the focus error signal FE=0 functions normally.
For example, in an example shown in FIG. 3B, the function of the focus servo loop for the grooves GB is made effective by giving a focus bias FB to the focus error signal FE so as to cancel the offset GFOF caused by the grooves GB.
In the case where the grooves GB/lands LD are traversed, however, the modulated signal becomes as illustrated. The larger the degree of modulation caused by the grooves GB/lands LD, therefore, the more the stability of the focus servo loop is hampered, resulting in a problem.
Furthermore, in recent years, there has been proposed a system using both the grooves GB and the lands LD as the recording and reproducing tracks for the purpose of increasing the recording capacity.
In the case of such a system, the servo function cannot be made effective by some fixed focus bias. Therefore, such a sophisticated and difficult control as to change over the focus bias according to whether the laser spot is currently in the grooves GB or in the lands LD is needed.
In other words, the following offset changeover processing is conducted. In the case where the focusing is conducted with respect to the lands LD, a focus bias for land for canceling the offset LFOF is given to the focus error signal FE. In the case where the focusing is conducted with respect to the grooves GB, a focus bias for grooves for canceling the offset GFOF is given to the focus error signal FE.
From these problems, it is demanded to make the degree of modulation caused on the focus error signal by the grooves GB/lands LD (or the degree of modulation caused on the focus error signal by a pit train) as small as possible. For example, in the system using either grooves or lands as the recording and reproducing tracks, it is demanded to minimize the stability hampering when a track is transversed. Furthermore, in the system using both lands and grooves as the recording and reproducing tracks, it is demanded to make the degree of modulation caused on the focus error signal by the grooves GB/lands LD, i.e., the offset difference, as small far as possible, and thereby make possible focus servo control to some degree without necessarily changing over the focus bias.